The monitoring and correction of tire non-uniformities are important aspects of the art of manufacturing pneumatic tires. Tire non-uniformities include dimensional variations, such as differences in the dimensions of the belts, beads, liners, and treads of the tires, differences in material properties (e.g. rubber stiffness), and flaws in the cosmetic appearance of the tires. Whatever the type of non-uniformity, they all result from variations in the manufacturing process. For example, dimensional variations may be caused by rubber flow in the tire molds, out-of-round curing bladders or tire molds, off-center loading at press, snaked treads or belts, tilted beads, tilted carcasses relative to belts, and tilted tires in the mold. Often, more than one source of variation may compound the extent of a non-uniformity; for example, non-uniform tire stiffness may result from both tire thickness variation and from variations in the stiffness property of the rubber itself.
Two dimensional non-uniformities of special importance are radial run-out and lateral run-out. Radial run-out is the variation in the tread radius of a tire, also referred to as tire "out-of-roundness". The primary production variations causing radial run-out are stretched components, wide component splices, grouped component splices, an out-of-round curing bladder, mold or building drum, eccentric carcass relative to belts, snaked chafer, and off-center loading at press. Radial run-out is monitored during tire production not only because it indicates dimensional variations, but also due to the fact that these non-uniformities contribute to certain force variations and tire imbalances, as discussed below.
Tire non-uniformities may cause one or more of the following effects on tire performance: force variations, imbalance forces and moments, conicity, ply steer and residual self-aligning torque. When non-uniformities are of a sufficient magnitude, the resulting force variations, imbalances, etc., will exceed an acceptable amount and the ride of the vehicle to which such tires are mounted will be adversely affected. As a tire rolls upon a surface, it transmits forces, for example, vehicle weight or centrifugal force, from the vehicle to which it is attached to that surface. Arising from these forces transmitted to the surface are equal and opposite reaction forces exerted by the road on the vehicle as transmitted through the tires.
Force variations are fluctuations in the magnitude of the forces which are exerted by a tire to a road surface on which it rolls, thus causing fluctuations in the reaction forces experienced by the vehicle. These variations in the forces are caused by differences in tire stiffness and/or geometry of the tire about its circumference or tread centerline and depend on which increment of the tire tread is contacting the road surface at a particular time. As an illustration of force variation, a perfectly round tire on a four wheel vehicle may be expected to transmit a constant force of 1/4 of the total weight of the vehicle to the surface, with the corresponding reaction force conveyed to the vehicle, as the vehicle travels. However, if there is a low spot on a tire (i.e. due to differences in radial run-out), during the time when that spot rolls over the surface, the distance from tire axis to the road surface at that corner of the vehicle will be slightly less than at the other three corners of the vehicle. This causes a decrease in the force exerted by the tire on the road, with a corresponding decrease in the reaction force transmitted to the vehicle, during the moment when the low spot is rolling over the surface. This decrease in the force exerted on the road and the reaction force transmitted to the vehicle is repeated during every revolution of the tire. The periodic fluctuation of the reaction force transmitted to the vehicle is experienced as a vibrational or acoustical disturbance of the ride of the vehicle.
Force variations include radial force variation, lateral force variation, and tangential force variation. Radial force variation is caused by radial run-out and variations in radial stiffness and is exerted in the radial direction of the tire, or in a direction perpendicular to the axis of rotation and non-tangential to the road surface. Radial force variation causes roughness to the vehicle ride at various speeds. Radial force variation is easily measurable by a variety of standard methods.
Tangential force variation is a more complex phenomena than the two discussed above. Tangential force variation, or fore-aft force variation, is experienced at the surface of contact between tire and road surface in a direction both tangential to the tire tread and perpendicular to the tire axis of rotation. Tangential force variations are very speed dependent and are experienced as a "push-pull" effect on a tire, which can be analogized to the sensation of a wheel barrow traveling over a bump in the road, i.e. increased force as the wheel barrow is pushed up the bump and decreased force as the wheel barrow travels down the bump. Investigations have shown that there are multiple mechanisms active in causing tangential force variation. However, to date, the tangential force variation are essentially unmeasurable on a typical production low speed tire uniformity machine, as discussed in more detail below, which operates at a speed, such as 60 revolutions per minute (RPM). Instead, tangential force variation can only be measured at highway speed or above, using a high speed laboratory tire uniformity machine, such as a Model HSU-1064, available from the Akron Standard Co. of Akron Ohio. Because of the low productivity and expense of the laboratory tire uniformity machine, the tangential force variation parameter can only be measured by sample methods. Still, tangential force variation does represent a tire uniformity characteristic for which 100% testing and correction would be preferred, if available.
In the usual tire manufacturing process, tires are placed first in a production tire uniformity machine to correct force variations and then placed in a tire balancing machine to check for unacceptable imbalance. A number of methods have been developed to correct excessive force variations by removal of rubber from the shoulders and/or the central region of the tire tread by means such as grinding. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of force measurements representative of the force exerted by the tire as these increments contact a surface. This data is then interpreted and rubber is removed from the tire tread in a pattern generated by this interpretation.
Force variation correction methods are commonly performed with a production tire uniformity machine (TUM), which includes an assembly for rotating a test tire against the surface of a freely rotating loading wheel. Typically, the tire is rotated at a low speed of about 60 rpm. In such an arrangement, the loading wheel is moved in a manner dependent on the forces exerted by the rotating tire and those forces are measured by appropriately placed measuring devices. When a tire being tested yields less than acceptable results, shoulder and center rib grinders are used to remove a small amount of the tire tread at precisely the location of non-uniformities detected by the measuring devices. As the tire is rotated, it is measured and ground simultaneously. In a sophisticated, low speed production tire uniformity machine, such as a Model No. D70LTX available from the Akron Standard Co. of Akron Ohio, the force measurements are interpreted by a computer and rubber is removed from the tire tread using grinders controlled by the computer. Examples of tire uniformity machines utilizing these methods are disclosed in U.S. Pat. Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284.